Joining a street team depends on what type of relevant skills you have to contribute to what ever it is that the team is involved in.
Prepare a CV of your relevant skills (for example: 'club/radio DJ, hardworking and wanting to get into the music biz') and forward it to the companies with street teams that you'd like to join... independent record promotion companies, in-house promo departments of record labels, college promotion, regional promotion etc.
It helps if you're presenting these companies with new areas or new ways for them to promote their product. Is your location an area they don't already cover? Have you identified any new ideas for promoting their product that they may have overlooked themselves?